Stabilization of magnesium hydroxide slurries

ABSTRACT

A treatment for reducing viscosity and inhibiting settling of an aqueous 30% or higher by weight magnesium hydroxide slurry which comprises adding to the slurry an amount, effective for the purpose of a combination of an alkyl sulfosuccinate surfactant, a xanthan gum and a clay.

BACKGROUND OF THE INVENTION

In the production of magnesium from sea water, one of the processingsteps produces a 30% or higher by weight magnesium hydroxide slurry.These slurries are unstable and rapidly separate or have a pasteconsistency and are very difficult to pump or flow through pipes due totheir high viscosity. This general technology is described in theKirk-Othmer Encyclopedia of Chemical Technology, Third Edition, vol. 14(1981), p. 634-635.

As noted, magnesium hydroxide slurries with a greater than about 30%weight magnesium hydroxide exhibit rapid viscosity increase and heavysettling, and are by nature extremely difficult to transport. Onepurpose for these materials involves feeding the slurry to boiler fueloil prior to ignition, in order to produce a softer scale that is easierto remove from the boiler. Consequently, the formation of a softer, morecompressible scale prevents damage to the boiler tubes which occursbecause of differences in thermal expansion coefficients between theboiler tubes and the deposit. Also, the feedlines bringing the slurry tothe furnace can often become clogged due to settling of the magnesiumhydroxide during the application. Therefore, a need arises to stabilizethe slurry in order to make it pumpable, and also to prevent settlingand clogging of feedlines.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for reducing viscosity andinhibiting magnesium hydroxide settling of an aqueous slurry withgreater than 30 wt. % magnesium hydroxide. The slurry is stabilized by atreatment comprising an alkyl sulfosuccinate surfactant, a water solublepolycarboxylate, and a rheology modifier (e.g., xanthan gum or magnesiumaluminum silicate clay). Preferred sulfosuccinate surfactants includesodium diisooctyl sulfosuccinate and adicarboxyethyl-n-octadecylsulfosuccinate, tetrasodium salt.

In a preferred embodiment of the present invention, samples forevaluation were prepared by dispersing the xanthan gum and clay inwater, followed by addition of the anionic surfactants, polymer andmagnesium hydroxide slurry. All samples were then evaluated forviscosity and stability (from 4°-50° C.).

The following results, as shown in Table I below, indicate that theunstabilized magnesium hydroxide slurry (control) completely separatedand hard settled within 48 hours, and exhibited viscosities greater than50,000 centipoises, or cps. Two particularly preferred embodiments ofthe present invention (Samples B and D) did not exhibit rapid settling,and viscosities remained low (less than 1200 cps) throughout the testperiod.

Preferred amounts of additives which effectively stabilize the magnesiumhydroxide slurry (not necessarily in combination) are about 0.05 to 0.2%xanthan gum, about 0.05-0.30% clay, about 0.1-2.0% alkylaryl sulfonatesurfactant (sodium dodecylbenzene sulfonate, or DDBSA), about 0.5-5.0%dicarboxyethyl-n-octadecyl sulfosuccinate, tetrasodium salt (DCEODS),about 0.5-5.0% diisooctyl sulfosuccinate (DOSS), and about 0.5-5.0%polyacrylic acid (PAA), the polyacrylic acid with a molecular weight offrom 1000-5000. The magnesium hydroxide slurries tested are allcommercially available.

                                      TABLE I                                     __________________________________________________________________________    Stabilization of Aqueous Magnesium Hydroxide Slurries                         (70 Wt. %)*                                                                                      Sample                                                     Material (wt. %)   Control                                                                            A   B   C   D                                         __________________________________________________________________________    Water              30.0 23.2                                                                              23.2                                                                              24.0                                                                              19.4                                      DOSS                    1.8 1.8 1.8 1.8                                       Clay                    0.2 0.2 0.2 0.2                                       Xanthan Gum             0.1 0.1 0.1 0.1                                       DCEODS                      3.8 3.8 3.8                                       PAA                     3.8         3.8                                       DDBSA                   0.8 0.8     0.8                                       Physical Properties                                                           pH                 9.8  9.8 10.1                                                                              9.9 9.9                                       Viscosities CPS @ 25° C.                                               Initial            1460 1120                                                                              600 600 600                                       7 days             >8000                                                                              3440                                                                              680 640 640                                       15 days            >8000                                                                              5560                                                                              860 640 600                                       30 days            >8000                                                                              >8000                                                                             980 800 600                                       Stabilities (4° C.)                                                    7 days (% separated (sep)/% sludged)                                                             50/100                                                                             0/0 0/1 0/0 0/0                                       15 days (% sep/% sludged)                                                                        50/100                                                                             2/0 2/2 2/5 0/0                                       30 days (% sep/% sludged)                                                                        50/100                                                                             5/0 5/2 10/15                                                                             1/0                                       Stabilities (50° C.)                                                   7 days (% sep/% sludged)                                                                         50/100                                                                             2/0 1/2 0/0 0/0                                       15 days (% sep/% sludged)                                                                        50/100                                                                             5/0 2/2 2/5 2/0                                       30 days (% sep/% sludged)                                                                        50/100                                                                             5/15                                                                              5/5 10/15                                                                             5/2                                       __________________________________________________________________________     *Silicone oilbased antifoam added to treatments.                         

While this invention has been described with respect to particularembodiments thereof, it is apparent that numerous other forms andmodifications of this invention will be obvious to those skilled in theart. The appended claims and this invention generally should beconstrued to cover all such obvious forms and modifications which arewithin the true spirit and scope of the present invention.

We claim:
 1. A method for reducing viscosity and inhibiting settling ofan aqueous 30% or higher by weight magnesium hydroxide slurry whichcomprises adding to said slurry an amount, effective for the purpose ofa combination of a dicarboxyethyl-n-octadecyl sulfosuccinate surfactant,a xanthan gum and a clay.
 2. The method as recited in claim 1 whereinsaid clay is a magnesium aluminum silicate clay.
 3. The method asrecited in claim 1 wherein the viscosity of the slurry is less thanabout 1200 cps.
 4. The method as recited in claim 1 wherein thecombination further comprises an alkylaryl sulfonate surfactant.
 5. Themethod as recited in claim 4 wherein said alkylaryl sulfonate surfactantis sodium dodecylbenzene sulfonate.
 6. A method for reducing viscosityand inhibiting settling of an aqueous 30% or higher by weight magnesiumhydroxide slurry which comprises adding to said slurry an amount,effective for the purpose of a combination of andicarboxyethyl-n-octadecyl sulfosuccinate surfactant, a water solublepolycarboxylate, a xanthan gum and a clay.
 7. The method as recited inclaim 6 wherein said clay is a magnesium aluminum silicate clay.
 8. Themethod as recited in claim 6 wherein the viscosity of the slurry is lessthan about 1200 cps.
 9. The method as recited in claim 6 wherein thecombination further comprises an alkylaryl sulfonate surfactant.
 10. Themethod as recited in claim 9 wherein said alkylaryl sulfonate surfactantis sodium dodecylbenzene sulfonate.